The disclosure of Japanese Patent Application No. HEI 9-279586 filed on Sep. 25, 1997 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to a power output device and a method of stopping a prime mover in the power output device and, more particularly, to a power output device controlling stoppage of the prime mover and a method related thereto.
2. Description of Related Art
Conventional vehicles or ships employ a power output device that includes a prime mover outputting power by means of fuel combustion and performs torque conversion to output a torque from the prime mover to a drive shaft. As such a power output device, a transmission combined with a fluid-type torque converter has been put into practice. The torque converter in this device is disposed between an output shaft of the prime mover and a rotational shaft coupled to the transmission and transmits power between both the shafts by transferring the fluid sealed in. This type of torque converter transmits power by means of fluid transfer, thereby generating a slippage between both the shafts, which causes a corresponding amount of energy loss. To be more precise, this energy loss is expressed as a product of a difference in of revolution speed between both the shafts and a torque transmitted to the output shaft, and is consumed as heat.
Thus, a vehicle equipped with such a power output device causes a problem of a large amount of energy loss in the torque converter, which leads to a low energy efficiency when the slippage between both the shafts becomes great, e.g. when the vehicle requires great power in taking off or in going up a slope at a low speed. Even in a steady running state, the power transmission efficiency in the torque converter does not reach 100%, so that a deterioration in fuel consumption is inevitable, e.g. in comparison with a manual transmission.
For this reason, some power output devices have already been proposed to replace the fluid-type torque converter. For example, the applicant proposed a power output device that is provided with a prime mover, a planetary gear constructed as a three-shaft type power input/output device, two electric motors and a battery (Japanese Patent Application Laid-Open No. SHO 50-30223). This power output device performs energy conversion of a power outputted from the prime mover and an electric power stored in the battery by means of the planetary gear and the two electric motors respectively, so as to output a desired power to a drive shaft. The applicant also proposed another power output device that is equipped with a prime mover, a planetary gear, two electric motors and a battery (secondary battery)(Japanese Patent Application Laid-Open No. HEI 10-98805, published Apr. 14, 1998). To stably output a desired power to a drive shaft, this power output device drive-controls the two electric motors such that the revolution speeds of three shafts constituting the planetary gear, i.e. a sun gear, a ring gear and a planetary carrier assume desired values respectively.
In these power output devices, the energy outputted from the prime mover is stored in the battery so that the thus-stored energy can be taken out whenever desired. Therefore, it is not absolutely necessary to maintain the balance between a power outputted to the drive shaft and a power outputted from the prime mover. On the contrary, what is desirable from the standpoint of an enhanced efficiency of the entire system is a construction wherein the prime mover is operated in a steady operating state that achieves a maximum efficiency, wherein if a surplus of energy exists relative to the power required for the drive shaft, that energy is stored as electric power and wherein if the battery is sufficiently charged, the prime mover is stopped so that the vehicle runs solely by means of power from the electric motors. In this case, the prime mover is intermittently operated.
However, it has been revealed by actually manufacturing a power output device having a torque conversion device interposed between the prime mover and the drive shaft that various problems are raised when the prime mover is intermittently operated. One of the problems is that torsional resonance may be caused due to a considerable weight of the torque conversion device coupled to the output shaft of the prime mover. It has also been revealed that the warm-up state of the prime mover or the like affects conditions for causing torsional resonance. Furthermore, the prime mover is connected with various peripheral devices including a catalytic converter. It has also been revealed that if the prime mover is operated or stopped independently of the states of those peripheral devices, some sort of inconvenience may be caused. For example, if the prime mover is stopped prior to completion of the warm-up process, the catalytic converter does not reach its activation temperature forever, which makes it inevitable to continue to drive the vehicle with an insufficient degree of exhaust gas purification.
In addition, the following problem has also been discovered. In the construction employing the planetary gear, part of the power outputted from the prime mover is directly outputted to the drive shaft via the planetary gear. Therefore, if fuel supply to the prime mover is cut off, the revolution speed of the output shaft of the prime mover also changes in accordance with a sudden change in the power outputted from the prime mover. Such a change in the revolution speed of the output shaft also affects the rotational shafts of the two electric motors via the planetary gear. The two electric motors are subjected to feedback control so as to eliminate a change in the revolution speed. However, since the power outputted from the prime mover changes earlier than the control of the electric motors, the drive shaft undergoes torque shock. The torque shock generated in the drive shaft is undesirable in terms of driving comfort.
Furthermore, the drive shaft of the vehicle may be coupled to other power control devices such as an antilock brake system (ABS) for preventing excessive slippage of wheels, a traction control system and the like. These control systems basically control driving forces applied to the wheels and provide the same effect as in the torque control of the drive shaft. Thus, from the standpoint of the control performed by the other power control devices, it is undesirable to cause fluctuations in a torque transmitted to the drive shaft by stopping the prime mover during such control. In this case, there arises a problem of control complication.